Well barriers such as completion isolation valves are well known. A requirement of any completion isolation valve is that the valve throughbore is maximised. Flapper valves are widely used as completion isolation valves because of their relatively compact structure. Compared to, for example, a ball valve, the reduction in the throughbore resulting from accommodation of the valve structure is minimised.
However, flapper valves have drawbacks. A conventional flapper valve only seals from one direction. To ensure bidirectional sealing, a device can be brought into engagement with the flapper valve to press the flapper against the seal seat. While this is effective, to operate the flapper valve and achieve a bidirectional seal, at least two and sometimes three control lines, such as hydraulic lines, are required.
Furthermore, conventional completion isolation and flapper valves require constant control pressure (where hydraulic pressure is used) to maintain the flapper valve in the open position. If the hydraulic pressure fails the valve closes which can cause interruptions to production or prevent intervention access to the well.